Sift the matcha powder into a small bowl, add the warm water, and whisk rapidly in a zig-zag motion using a bamboo whisk or frother until dissolved and foamy.

Combine the ripe banana, oat milk, and optional maple syrup in a blender, processing on high speed until completely smooth and frothy.

Fill a tall serving glass to the top with ice cubes.

Pour the blended banana milk over the ice, leaving roughly one inch of headspace at the top of the glass.

Gently pour the whisked matcha over the back of a spoon onto the banana milk to form a clean, floating top layer, then serve immediately.